PHYSICAL CONDITIONS OF FOLSOMIA CANDIDA OCCURRENCE IN SHALLOW GROUNDWATER

Folsomia candida (Fc) is a soil arthropod. Fc usually occurs in leaf litter or shallow soils, is found in all parts of the world and has been established as a standard test organism for evaluating ecotoxicity of soils. Fc has been used in toxicity studies of metals, herbicides, fungicides and various insecticides and is becoming the “white laboratory rat” of soil ecosystems. This presentation reports on recent work from an ongoing investigation of Fc occurrence in wells penetrating a shallow (2.5 to 5.7 m. below grade) lake-plain aquifer along the southwestern coast of Michigan, USA. Fc occurrence in groundwater is uncommon to rare, or at least has been under-reported in the hydrogeology and entomology literature.

Fc occurs in some wells, but not others in the study area. Questions investigated regarding Fc occurrence were: 1) Is pore space volume a limiting factor in whether or not Fc can occur at particular location? 2) Does Fc reside in the saturated pore spaces or in the unsaturated spaces directly above the water table/capillary fringe? The first question was addressed by multiple soil borings from surface to water table in close proximity to wells which contained abundant Fc, and wells which contained few to no insects. Composite samples (3-15 cm thick) collected from a depth of 1 meter above the water table down to the water table were dried at 120^oC and sieved with ASTM standard sieves. Effective grain size (d₁₀), median grain size (d₅₀) and uniformity coefficient (d₆₀/d₁₀) were determined for each composite. Variable cubic packing relationships, involving grain size and uniformity coefficient, were used to estimate the amount of pore volume space available compared to the average body volume of an individual Fc. Preliminary findings indicate that small differences in representative grain size (d₁₀ or d₅₀) can determine feasibility of Fc occupancy, but do not accurately explain the presence or absence of Fc in the study area. The second question was addressed by examining water level elevations and screen placement. A positive relationship exists between length of screen above water table and Fc abundance, indicating that Fc enters the wells from the unsaturated zone.

The authors suggest that hydrologists studying similar shallow unconfined aquifers be on the “look out” for the presence Fc, or other insects in the ground water.